The present invention relates to a brake system with slip control provided for automotive vehicles with front-wheel drive or all-wheel drive which is equipped with an auxiliary-energy-supplied braking pressure generator. The wheel brakes of a front wheel and of a rear wheel are respectively connected to the generator by way of two hydraulically isolated pressure-medium circuits containing inlet valves switched to their opened condition when inactive. The system further comprising return lines connecting the wheel brakes with a pressure-compensating and supply reservoir and containing utlet valves closed in their inactive position. Wheel sensors are provided for the determination of the wheel rotational behavior as well as electric circuits for the processing of the sensor signals and for the generation of braking pressure control signals which are adapted to be delivered to the inlet valves and the outlet valves.
As is known, locking of the vehicle wheels during a braking operation can be prevented in hydraulic or compressed air-driven brake systems with the aid of so-called inlet and outlet valves which are inserted into the pressure-medium conduit to the wheel brakes or into a return line or a pressure-discharge conduit and which allow to stop further rise of the braking pressure or to reduce the braking pressure as soon as the deceleration and/or the wheel slip exceeds a predetermined value. Systems of this type are provided in variations.
An optimal braking behavior (i.e., high driving and directional stability and, at the same time, a short stopping distance) can be expected if the braking pressure is controlled individually at each vehicle wheel. However, since the expense required for the components including control circuitries, monitoring and safety circuitries, etc. is very high, limitation to two or three control channels in connection with specific criteria such as "select-low, select-high" and with concurrent control of the braking pressure in several wheel brakes results in impairment of the braking behavior because it is expedient to depart from the individual control of all wheels.
Therefore, dual-circuit hydraulic brake systems of the type described have been developed and described, wherein, on detection of an imminent locked condition, one common pair of inlet and outlet valves permits variation of the braking pressure concurrently, jointly for the two wheels connected. If, in this arrangement, the pressure in each control channel is dimensioned according to the select-low principle taking into account the wheel with the worst road contact, a longer stopping distance must be tolerated.
Furthermore, in such brake systems it is known to additionally insert into the pressure-medium conduit to the wheel brake at the rear wheel a two-way/-two position directional control valve. Accordingly, when the rear wheel is relieved from load due to the dynamic axle load shift, locking of the unloaded rear wheel is prevented after change-over of said valve and that, simultaneously, further braking pressure rise at the front wheel can be performed (German patent application No. 31 36 616). In situations in which it is important to make use of the braking effect of the rear wheels, in particular in the presence of a low adherence value, this known measure will not provide any improvement.
Moreover, it has been proposed to derive the reference input for the dimensioning of the braking pressure in the diagonals from the front wheel, but to perform a change-over in certain situations according to predetermined criteria so that the rear wheel instead of the front wheel will temporarily take the lead, that is to determine the braking pressure (German patent document No. 33 14 802.3). Nevertheless, in some situations, joint control of the braking pressure in both wheels connected to a pressure medium circuit is a disadvantage compared to systems with three or four control channels.
It is known to economize control channels in the case of a diagonally split-up dual-circuit brake system by providing one single pressure modulator per pressure-medium circuit; and, for better adaptation of the braking pressure control to the respective situation, to temporarily apply the selection criteria select-low/select-high, in response to which the control of the pressure of the two circuits is effected, to specific groups (European Pat. No. 51 801). In this arrangement, locking of the least loaded wheel is tolerated for a short stopping distance.
It is therefore an object of the present invention to overcome the shortcomings of the known slip-controlled dual-circuit brake systems and to provide a brake system which is economical to manufacture and which, nevertheless, affords effective braking with slip control both at a high and a low friction values in all practical situations, while driving stability and steerability are maintained.